Photolithography plays a critical role in the art of printed circuit packaging. Photolithography is used to define in a thin film of photoresist those regions from which copper is to be selectively etched to subtractively form circuitization, or to which copper is selectively plated to additively form circuitization. Photolithography is also used to personalize soldermasks and dielectric layers.
There are basically two types of photoresist: negative acting and positive acting. Positive photoresists and negative photoresists are both formed from monomers, hereinafter referred to as "monomeric units", such as, for example, acrylates and ethers of bisphenol A. Examples of monomeric units used in the conventional photoresists are as follows: t-butyl acrylate, 1, 5 pentanediol diacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate, 1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethylene glycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycol diacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanediol diacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate, tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, polyoxyethylated trimethylolpropane triacrylate and trimethacrylate and similar compounds as disclosed in U.S. Pat. No. 3,380,831, 2,2-di-(p-hydroxyphenyl)-propane diacrylate, pentaerythritol tetraacrylate, 2,2-di(p-hydrohyphenyl)-propane dimethacrylate, triethylene glycol diacrylate, polyoxyethyl-2,2-di(p-hydroxyphenyl)-propane dimethacrylate, di-(3-methacryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-methacryloxyethyl) ether of bisphenol-A, di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A, di-(2-acryloxyethyl) ether of bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A, di-(2-methacryloxyethyl) ether oftetrachloro-bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl)etheroftetrab romo-bisphenol-A, di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A, di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol, di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid, triethylene glycol dimethacrylate, polyoxypropyltrimethylol propane triacrylate, ethylene glycol dimethacrylate, butylene glycol dimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetriol trimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate, pentaerythritol trimethacrylate, 1-phenyl ethylene-1,2-dimethacrylate, pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate, 1,5-pentanediol dimethacrylate, diallyl fumarate, 1,4-benzenediol dimethacrylate, 1,4-diisopropenyl benzene, and 1,3,5-triisopropenyl benzene.
In addition to the reactive monomeric units mentioned above, the photoimageable compositions used to form negative and positive photoresists can also contain one or more free radical-initiated and polymerizable species with molecular weight of at least about 300. Monomeric units of this type are an alkylene or a polyalkylene glycol diacrylate and those described in U.S. Pat. No. 2,927,022. Particulate thickeners such as, for example, silicas, clays, alumina, bentonites, kalonites, and the like can also be used in the photoimageable compositions. Dyes and pigments may also be added to increase the visibility of the resist image. Any colorant used however, should be transparent to the actinic radiation used to polymerize the monomeric units. With some compositions, it is desirable to add a plasticizer, either solid or liquid, to give flexibility to the film or coating. Generally inert solvents which are volatile at ordinary pressures are used to prepare these photoresist compositions.
During processing of the photoresist, a photoimageable film is first applied to a circuit board and then patterned by exposure of preselected regions to actinic radiation. To develop the resulting pattern of polymerized and unpolymerized material, the coated board is contacted with a liquid developer either by dipping or spraying. The commonly assigned U.S. Pat. No. 5,268,260 of N. R. Bantu, Anilkumar Bhatt, Ashwinkumar Bhatt, G. W. Jones, J. A. Kotylo, R. J. Owen, K. I. Papathomas, and A. K. Vardya for Photoresist Develop and Strip Solvent Compositions and Method for Their Use, incorporated herein by reference, describes the use of the low vapor pressure, high boiling solvents, benzyl alcohol, propylene carbonate, and gamma butyrolactone for developing and stripping acrylate-based photoresist such as DuPont Riston T-168 or photoimageable dielectric material.
In the case of negative acting photoresists, the unpolymerized material used to form the photoresist, i.e., the monomeric units of acrylate or epoxy, is dissolved in the developer at low temperature, preferably between 15.degree. C. and 45.degree. C. The dissolved material, which consists primarily of monomeric units of the acrylate or epoxy, and the developing solution are then removed from the board by allowing the solution to run off into a containment tank. To further enhance development of the pattern, the residual dissolved material and developing solution are rinsed from the board, preferably with warm water. High vapor pressure organic solvents, such as isopropyl alcohol, acetone, methyl ethyl ketone and xylene, may also be used as a rinse. Positive acting resists behave oppositely. Actinic radiation renders the positive acting photoresist more soluble in the developer, and the exposed regions are removed preferentially by the developer. The effluent produced by this process is an impure solution of developer, which is laden with monomeric units and other impurities. Typically, the effluent of such process contains greater than 10 percent weight of monomeric units.
Following circuitization of the board, the polymerized photoresist may be stripped from the board, preferably by spraying with a stripping solution at elevated temperatures, preferably between 50.degree. C. and 100.degree. C. The commonly assigned U.S. Pat. No. 5,268,260 of Bantu et al also describes the use of benzyl alcohol, propylene carbonate, and gamma butyrolactone for stripping acrylate-based photoresist such as DuPont Riston T-168. The stripping solution causes the polymerized photoresist to swell, and debond, i.e., detach from the underlying substrate, and flake off of the board. The stripping can be assisted by gentle scrubbing with brushes. The resist particles and stripping solution are then removed from the board, preferably by flushing into a containment tank. Any residual polymerized film particles and stripping solution are rinsed from the package, preferably at elevated pressures, preferably with warm water. The water rinse can be replaced by rinsing in high vapor pressure solvents. The effluent produced by this process is an impure solution of stripper, laden with dissolved polymeric photoresist, suspended particles of polymeric photoresist and other impurities. Typically, such effluent contains less than 10 percent by weight monomeric units, and possibly as little as zero weight percent monomeric units.
Large volumes of liquid waste containing an impure low vapor pressure, high boiling solvent result from the above-described processes. This liquid waste must be further processed prior to release into the environment, as by incineration. Such methods of dealing with the liquid waste are costly. Moreover, the costs to the industry in terms of purchasing virgin material, i.e., pure benzyl alcohol, pure gamma butyrolactone, and pure propylene carbonate, and the costs to the environment of manufacturing virgin material are significant.
Accordingly, it is desirable to have a new method of reducing the amount of liquid waste that results from these and other industrial processes. A method that permits recovery of relatively pure benzyl alcohol, propylene carbonate, or gamma butyrolactone which can then be re-used by the industry, and thus prevent the need to purchase virgin material, is especially desirable.